This invention relates generally to the use of focused acoustic energy in the ejection of fluids, and more particularly relates to acoustic ejection of fluid droplets using a large F-number focusing element.
A number of patents have described the use of acoustic energy in droplet ejection. For example, U.S. Pat. No. 4,308,547 to Lovelady et al. describes a liquid drop emitter that utilizes acoustic principles in ejecting liquid from a body of liquid onto a moving document for forming characters or bar codes thereon. Lovelady et al. is directed to a nozzleless inkjet printing apparatus wherein controlled drops of ink are propelled by an acoustical force produced by a curved transducer at or below the surface of the ink.
The Lovelady et al. patent makes use of a piezoelectric shell transducer to both generate and focus the acoustic energy. Several other methods have also been developed to focus the generated acoustic energy and eject a droplet of liquid. For example, acoustically illuminated spherical acoustic focusing lenses as described in U.S. Pat. No. 4,751,529 to Elrod et al. and planar piezoelectric transducers with interdigitated electrodes as described in U.S. Pat. No. 4,697,105 to Quate et al. The existing droplet ejector technology has been used in designing various printhead configurations, ranging from relatively simple, single ejector embodiments for raster output scanners (ROS""s) to more complex embodiments, such as one or two dimensional, full page width arrays of droplet ejectors for line printing. It has also found use in the synthesis of arrays of biological materials, as described in co-pending, commonly assigned applications Ser. No. 09/669,996, xe2x80x9cACOUSTIC EJECTION OF FLUIDS FROM A PLURALITY OF RESERVOIRS,xe2x80x9d filed Sep. 25, 2000, Ser. No. 09/727,392, xe2x80x9cFOCUSED ACOUSTIC ENERGY IN THE PREPARATION AND SCREENING OF COMBINATORIAL LIBRARIES,xe2x80x9d filed Nov. 29, 2000, and Ser. No. 09/765,947, xe2x80x9cHIGH THROUGHPUT BIOMOLECULAR CRYSTALLIZATION AND BIOMOLECULAR CRYSTAL SCREENING,xe2x80x9d filed Jan. 19, 2001.
However, the development of nozzleless fluid ejection has generally been limited to ink printing applications and has relied exclusively upon acoustic lenses having F-numbers of approximately 1. Unfortunately, low F-number lenses place restrictions on the reservoir and fluid level geometry and provide relatively limited depth of focus, increasing the sensitivity to the fluid level in the reservoir. For example, in bimolecular array applications the various bimolecular materials from which the array is constructed are usually contained in individual wells in a well plate. These wells often have aspect ratios of approximately 5:1, i.e., the wells are five times as deep as their diameter. The narrowness of the wells requires that when F1 lenses are used the surface of the fluid within the reservoir be no further from the lens than the width of the lens aperture. Therefore, when using an F1 lens in a 5:1 aspect ratio well, only the bottom fifth of the reservoir may be filled with fluid.
Thus, there is a need in the art for improved acoustic fluid ejection devices and methods having sufficient droplet ejection accuracy so as to enable preparation of high-density molecular arrays without the disadvantages associated with low F-numbered lenses. While the use of F2 lenses has been suggested in Elrod et al. (1989), xe2x80x9cNozzleless droplet formation with focused acoustic beams,xe2x80x9d J. Appl. Phys 65(9):3441-3447, the reference indicates that such lenses provide unpredictable results in terms of droplet diameter and usable depth of focus. Surprisingly, it has now been found that larger F-numbered lenses provide additional advantages over F1 lenses as the use of lenses having F-numbers greater than 2 allows for far greater control over droplet size and velocity while providing greatly enhanced depth of focus.
Accordingly, it is an object of the present invention to provide devices and methods that overcome the above-mentioned disadvantages of the prior art. In one aspect of the invention, a device is provided for acoustically ejecting a plurality of fluid droplets toward a designated site on a substrate surface, comprising: a reservoir adapted to contain a fluid having an aperture that enables conduction of acoustic energy in a substantially uniform manner, said aperture having an effective dimension; and an ejector comprised of an acoustic radiation generator for generating acoustic radiation and a focusing means capable of focusing the generated acoustic radiation to emit a droplet from a surface of a fluid contained within the fluid reservoir said surface being an effective distance from the aperture, wherein the ratio of the effective distance to the aperture to the effective dimension of the aperture is greater than about 2:1. The device may further comprise a means for positioning the ejector in acoustic coupling relationship to the reservoir. Preferably, the ratio is greater than approximately 3:1, or even greater than about 4:1. The device may also comprise a plurality of reservoirs each adapted to contain a fluid, and wherein the device is capable of ejecting a fluid droplet from each of the plurality of reservoirs toward a plurality of designated sites on the substrate surface.
In another aspect, the invention relates to a method for ejecting fluids from fluid reservoirs toward designated sites on a substrate surface. The method involves providing a device comprised of a reservoir containing a first fluid, said reservoir having an aperture that enables conduction of acoustic energy in a substantially uniform manner, said aperture having an effective dimension and an ejector comprised of an acoustic radiation generator for generating acoustic radiation and a focusing means capable of focusing the generated acoustic radiation to emit a droplet from a surface of the first fluid contained within the fluid reservoir said surface being an effective distance from the aperture, wherein the ratio of the effective distance from the aperture to the effective dimension of the aperture is greater than about 2:1. The ejector is then positioned so as to be in acoustically coupled relationship to the fluid-containing reservoir, so that the position of the ejector places the focal point of the ejecting means near the surface of the first fluid, and hence, the effective distance from the aperture. Finally, the ejector is activated, thereby generating acoustic radiation having a focal spot of a diameter D at the surface of the first fluid, resulting in the ejection a droplet of the first fluid from the reservoir. If desired, the method may be repeated with a plurality of fluid reservoirs each containing a fluid, with each reservoir generally although not necessarily containing a different fluid. The acoustic ejector is thus repeatedly repositioned so as to eject a droplet from each reservoir toward a different designated site on a substrate surface. In such a way, the method is readily adapted for use in generating an array of molecular moieties on a substrate surface.